Transmitter



TRANSMITTER Filed Oct. 29,

H. D. ALBRECHT Dec. 24, 1940.

Gttomeg Patented Dec. 24, 1940 UNITED STATES 2,226,214 j TRANSMITTERHarold D. Albrecht, Pennsauken, N. J., assigner to Radio Corporation ofDelaware America, a corporation of Application October 29, 19.48, SerialNo.'2.37,637

y Claims.

My invention relates to vacuum tube coupling systems, and moreparticularly to modulation circuits in a transmitting system and inversefeedback circuits. i

In transmitting systems and amplifiers femploying degenerative orinverse feedback circuits 'for reducing distortion and hum, considerablediiculty has been experienced from phase shift in the feedback loop,particularly incases where audio-frequency transformers have been usedin the circuit. It has been common practice to employ a conventionalpush-pullv transformer consisting of a split primary and a closelycoupled secondary for coupling from a push-pull or class B high poweredmodulation amplifier to an R F. amplifier anode circuit in a radiotransmitter, for modulating the R. F. carrier in accordance withimpressed audiofrequency signal waves. This transformer has preventedrealizing the full benefits of feedback in the reduction of distortionand hum, although in some cases special phase correcting networks havebeen added for the purpose of reducing the phase shift.

It is, accordingly, an object of my invention to provide, in atransmitting system, improved coupling means free from phase shift, forcoupling a modulator to an R. F. amplifier or relay employingdegenerative feedback in the system. Y

It is a further object of my invention to provide an improved circuitarrangement for coupling a double ended or divided output amplifier orrelay into a single ended circuit.

It is a still further object of my invention to provide, in an amplifiersystem having degenerative feedback, a simplified inductive couplingmeans with greatly reduced phase shift for coupling a double ended ordivided output amplifier to a single ended circuit.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and its method ofoperation, together with lfurther objects and advantages thereof, willbest be understood by reference tothe following description of ayspecific embodiment, taken with the accompanying drawing, diagrammaticview of a radio transmitting system embodying my invention.

Referring to the drawing, I have shown schematically an oscillator I, anR. F. amplifier 3, both in block diagram, coupled to the input of apush-pull R. F. amplifier stage 5, which in turn is coupled in push-pullto a final high powered push-pull R. F. amplifier 1. 'Ihe amplifiertubes in which the single figure is a include the usual filament 2, grid4 and anode 6. The output of amplifier 1 is coupled through transformer9 to the antenna I I. The amplifiers 5 and 1 are coupled by means of avariably tuned circuit I3 including a split tuning capacitor I5 andcenter-tap R. F. inductance coil I1.

The R. F. amplifier 5 is modulated by means of a class B amplifier stageI9, in accordance with the teachings of Barton Patent 2,063,290 andincludes tubes 2I2I and 23-23 arranged in parallel push-push relationrespectively. The outputs of tubes 2I and 23 are connected to a centertapped iron core chokev coil 25 in balanced relation. In effect, coil 25is an auto transformer, the two halves being closely coupled so that thevoltage across one half is practically the same as that across the otherhalf, although 180 out of phase. One end terminal of coil 25 isconnected, through a stopping capacitor 21, e. g., of the order of 8mfd., to a modulating reactor 29 at its high potential end, preferablyno magnetic coupling existing between the reactor and the coil 25.

The reactor is connected in the usual manner for plate modulation,between the high voltageA anode supp-ly source 3I and the anodes orplates 6 of push-pull tubes 5, through an R. F. cho-ke 33 and resistor35, to a center tapped terminal on the coil I1. It will be noted thatthis terminal on coil I1 is at ground potential for the R. F., and thatthis point is varied at audio potential with respect to ground, by meansof the modulator. The anodes of tubes 5 are effectively in parallel sofar as the impressed audiofrequency is concerned, and for this reasonare considered to be 'a single ended circuit into which the dividedoutput of the modulator I9 feeds. It is within the scope of my inventionto employ my auto transformer to couple the output from a push pullaudioamplifier into the grid circuit of a conventional single endedaudioamplifier. I am well aware of the fact that an auto transformerconnection of somewhat similar nature has been employed heretofore infeeding from a single ended output stage into a push-pull stage, asshown 'and claimed by Davis Patent 2,032,921, but I am not aware of anyprior arrangement for coupling in -a reverse manner with resultingimprovements, and more particularly in an inverse feedback combination,in accordance with my invention. It is also withinthe scope of myinvention to employ the arrangement of the Davis patent in a feedbackloop circuit for reduction of phase shift.

In the system shown, which has been built and used in accordance with myinvention, the amplifier I3 is of the class B type, operated With gridbias, as disclosed in patent of Loughren 1,699,110, although other typesof amplifiers, e. g., class A and class AB, could be employed inaccordance with my invention. The tubes 2l and 23 were of the lowimpedance type, but were provided with sufficient negative grid bias soas not to be driven to draw grid current during normal operation,although an amplifier in which grid current is drawn could be employed;for example, as shown in Barton Patents 2,084,180 and 2,084,181. Theamplifier 5 as actually used, was a class C type, although here againother types could be employed.

Ahead of power amplifier stage I9 is an audioamplifier, shown in blockdiagram at 3l. For the purpose of providing inverse or negativefeedback, a portion of the final output energy existing in transformer 9is taken, by means of a coupling coil 39, and fed back, through arectifier circuit 4I into the A. F. amplifier 3l, in accordance with theteachings of DeWitt Patent 2,093,751.

While I have illustrated my invention inconnection with this type offeedback circuit, it also has advantages when used in a feedbackinvolving the audio amplifier only, for example, as illustrated in Omanapplication, Serial No. 153,574., filed July 14, 1937 (same assignee).It will be observed that in the Oman application the output audiofrequency transformer is not included in the feedback loop. My inventionWill permit the inclusion of the output coupling means in the feedbackloop by virtue of the reduced phase shift.

Besides eliminating the transformer, and the corresponding difficultiescaused by phase shift, I have found that the use of a choke coil issubstantially less expensive and simplifies design.

'40 My improvement was applied in a given case to an existingtransmitter, in use in a broadcasting station, and it was found that thephase shift was reduced to such an extent that the distortion in thetransmitter, at 7500 cycles, was reduced to 45 approximately half of itsformer value. In making this change, I disconnected the secondary of theexisting audio transformer of the system, using the primary as a chokecoil, and added a modulating reactor in the manner shown at 29 50 in thepresent drawing.

Although I have illustrated but one embodiment of my invention, othermodifications will be apparent from the disclosure to those skilled inthe art. My invention, therefore, is not to 55 be limited except insofaras is necessitated by frequency amplifier characterized by a doubleended output circuit balanced for radio frequency currents, an audiomodulation amplifier having output electrodes connected in a doubleended output circuit, a reactor connected to said electrodes andconstituting an auto-transformer for coupling said audio output circuitto said radio frequency amplifier output circuit in single endedrelation for audio frequency modulation,

and a degenerative feedback loop circuit in said system including saidauto transformer.

2. In a transmitting system, a radio frequency amplifier, a double endedoutput audio amplifier, said amplifiers including thermionic tubeshaving cathodes, control electrodes and output electrodes, a powersupply circuit for said output electrodes, means for coupling said audioamplifier to the radio frequency amplier in single ended relationcomprising a choke coil connected in balanced relation to the outputelectrodes of said audio frequency amplifier, a modulating reactor andstopping capacitor, and a coupling connection through said stoppingcapacitor to a point on said modulating reactor, said reactor beingconnected in the output electrode power supply circuit to said radiofrequency amplifier, and said choke coil and reactor being characterizedby absence of magnetic coupling, whereby phase shift is reduced.

3.` The invention as set forth in the next preceding claim, wherein adegenerative feedback circuit is connected from the output of said radiofrequency amplifier to the input of said audio amplifier.

4. In a transmitting system, a radio frequency amplifier connected toy asource of radio frequency current, a double ended audio frequencyamplifier connected to a source of audio frequency current andcomprising a pair of thermionic devices connected in balanced relation,said devices having output electrodes, a power supply circuit connectedto said radio-frequency amplifier, an audio frequency reactor having endterminals connected respectively to said output electrodes, means forconnecting a portion of said reactor to said power supply circuit insuch manner that said reactor constitutes an auto transformer formodulating said radio frequency amplifier with audio frequency currentin said reactor, and an inverse feedback circuit connected from theoutput of said radio frequency amplifier to the input of said audiofrequency amplifier and forming therewith a feedback loop includingtherein said reactor.

5. The invention as set forth in claim 4 wherein said audio frequencyamplifier is of the class B type.

HAROLD D. ALBRECHT.

